Short-Lived Climate Pollutants in the Fifth IPCC Assessment Report Working Group I The Physical Science Basis

The Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) provides the latest comprehensive evaluation of the factors driving climate change. Regarding short-lived climate pollutants (SLCPs), AR5 finds larger contributions to climate change from methane and carbonaceous aerosols than the previous IPCC assessment while noting that uncertainties in the influence of aerosols remain large.
The AR5 highlights the difference in the climate impacts of SLCPs and long-lived gases (such as carbon dioxide (CO2). It reports that although simple metrics are commonly used to assess the relative impact of different pollutants (e.g. under the Kyoto Protocol), metric “values are very dependent on metric type and time horizon”, especially in comparing SLCPs and CO2. The AR5 therefore does not endorse any single metric or time horizon as preferable for characterizing the impacts of pollutants, instead concluding that impact analyses need to consider multiple endpoints, such as both near-term and long-term climate change and rates of climate change. These conclusions imply that SLCPs should not be traded against CO2 as any method to establish ‘equivalence’ in a particular impact would not hold for other impacts, and they support the CCAC’s focus on SLCPs as complimentary to, but not substituting for, actions to reduce CO2 emissions.
The AR5 also features a Frequently Asked Question (FAQ 8.2) “Do Improvements in Air Quality Have an Effect on Climate Change?” It concludes that they do, and illustrates the effects that both ozone and particulate matter pollution controls may have on climate. It also notes “controls on anthropogenic emissions of methane to lower surface ozone have been identified as ‘win–win’ situations”, with the ‘wins’ referring to improved air quality and reduced climate change. These conclusions broadly support the foci of the CCAC.